1. Field of the Invention
The present invention relates to a silicon nitride sintered body excellent in mechanical strengths particularly at room temperature, and also in productivity and cost, and a process for producing the same.
2. Description of the Prior Art
Regarding the silicon nitride material, various research and development have hitherto been made mainly on sintering method, sintering aid and limitation of a contained crystal phase, etc., for the purpose of improving the strength. For example, as regards the sintering method, the hot press sintering has realized a bending strength of about 100 kg/mm.sup.2 [see Am. Ceram. Soc. Bull., vol. 52, p. 560 (1973)] and use has been made of hot isotatic pressing (HIP) in a glass capsule. These methods provide a sintered body having excellent strength properties but are not always excellent in the productivity and cost. In order to solve such a problem, a proposal has been made on gas pressure sintering [see, for example, Mitomo, Funtai to Kogyo (Solids Handling Processing Industry), vol. 12, No. 12, p. 27 (1989)]. In this method, however, the densification of the final sintered body companies the growth of .beta.-Si.sub.3 N.sub.4 crystal grains and, thus, is highly liable to bring about the deterioration of the strength due to the precipitation of coarse crystal grains. Further, since the sintering is generally conducted under a nitrogen gas pressure of 10 atm or more, large-size sintering equipment becomes necessary as in the case of the hot pressing and HIP. For this reason, this method has not been regarded as a technique capable of sufficiently satisfying both the requirements of properties and productivity. Regarding the sintering aid, Japanese Patent Publication Nos. 21091/1974 and 38448/1973 disclosed silicon nitride sintered bodies comprising Si.sub.3 N.sub.4 -Al.sub.2 O.sub.3 -Y.sub.2 O.sub.3 wherein Y.sub.2 O.sub.3 was used as a main aid. As described therein, it is conceivable that a .beta.-type silicon nitride (.beta.-Si.sub.3 N.sub.4) crystal grain forms a fibrous structure in the sintered body and dispersed in the matrix, thus improving the strength and toughness. That is, this method positively utilized the fact that the .beta.-Si.sub.3 N.sub.4 crystal has a hexagonal crystal structure and therefore grows anisotropically in the direction of the C-axis. In particular, as described in the Japanese Patent Publication No. 38448/1973 and Journal of the Ceramic Society of Japan, vol. 94, p.96 (1986), a fibrous .beta.-Si.sub.3 N.sub.4 often grows in a length of ten-odd .mu.m or more in the direction of the C-axis. In this technique as well, however, there is a possibility that the grain growth may bring about an abnormal growth and the formation of pores, thus lowering the strength. Further, in a sintered body produced through the use of a sintering aid alone in this method, no sufficient densification can be attained without raising the sintering temperature to 1700 to 1900.degree. C., and in the nitrogen gas pressure sintering near atmospheric pressure, silicon nitride decomposes through sublimation, so that no stable sintered body can be obtained in some cases. Therefore, similarly, this method cannot be regraded as excellent in both the properties of the sintered body and the cost. In any of the above-described methods, the strength of the resultant sintered body is about 100 kg/mm.sup.2 at the highest in terms of the three-point bending strength according to JIS R 1601, and no satisfactory properties can be always obtained when the use of silicon nitride materials in various applications is taken into consideration.